The field of test and measurement spans a wide range of apparatus and techniques that include stages designed for accurate positioning and moving of samples under test. More specifically, the field of optical metrology requires stages that are capable of moving samples in a way that is accurate, reproducible and allows the inspection of the entire surface area of the sample. In other words, the stage needs to allow the optical inspection mechanism to access with its probe beam any surface portion of the sample. This is particularly important in samples that are disk-shaped, such as semiconductor wafers, whose features are being examined or inspected prior to dicing or cutting the wafer into individual circuit dies.
The prior art teaches a number of stages for retaining and moving samples, such as wafers for optical inspection. For example, U.S. Pat. No. 6,320,609 teaches an R-θ stage and assumes that the center of rotation of the polar coordinate stage can be made coincident with the center of measurement beam spot (or the center of the field of view of the imaging system). In practice, there is always some offset between these two centers. This means that there is an area, i.e., a blind spot that cannot be placed under the beam spot by an R-θ stage. In addition, when grating structures on a wafer are measured at oblique incidence of the test beam, the grating orientation relative to the direction of incidence varies from location to location, because the wafer will typically rotate as it moves. This is a serious problem for optical scatterometry measurements.
To overcome this problem, U.S. Pat. No. 6,882,413 teaches a very complicated optical setup. The setup is configured to move the optical assembly in a manner similar to the way in which rotation is compensated. Unfortunately, due to the small amounts of rotation, or even continuous rotation, the apparatus taught in this reference suffers from loss of angular resolution. Furthermore, misalignments in wafer positioning cause a blind spot that cannot be examined. A similar problem affects a system for measuring periodic structures, as disclosed in U.S. Pat. No. 6,721,052 by Zhao et al. Here the inventors are studying a periodic structure by illuminating it by polychromatic electromagnetic radiation and collecting radiation from the structure in two different polarizations. To reduce the footprint of the system, the measurement instrument and the wafer bearing the periodic structure are both moved. For example, they both undergo translational and rotational motion in such a way that the illumination beam from the apparatus scans a spiral path on the wafer. Thus, the system incurs the problems associated with continuous rotation as discussed above.
In fact, none of the above approaches offer a simple and effective apparatus and method for examining disk-shaped samples such as semiconductor wafers with a small footprint stage that is not susceptible to blind spots and issues associated with low angular resolution.